1. Field of the Invention
The present invention is directed to the field of liquid level sensors and more specifically to the area of sensors that detect changes of liquid level about a predetermined level.
2. Description of the Prior Art
Liquid level sensors containing switches to indicate whether a sensed liquid is above or below a predetermined level are numerous in the prior art.
A liquid level system for detecting the level of vehicle engine oil is disclosed in U.S. Pat. No. 3,939,470, wherein thermistors are located at opposite sides of an oil reservoir exposed to the oil therein. The system detects variations in thermal dissipation at the thermistors when they are immersed in oil or exposed to the air above the oil. While the sensors themselves appear to be simple in construction, the system itself requires a rather complex circuit to monitor the sensors and make the determination as to whether the oil level has dropped below the predetermined level.
Another type of liquid level sensor is disclosed in U.S. Pat. No. 3,750,124 which utilizes a more complex sensor than that described in the above-noted patent and a relatively simple circuit to determine when the liquid level has dropped below a predetermined level. The system is shown incorporated in a vehicle brake master cylinder reservoir and utilizes an electrical reed switch sensor mounted horizontally in the vertical wall of the reservoir. The sensor incorporates a permanent magnet encapsulated in a float surrounding the reed switch. Whenever the liquid level is above the predetermined level, the float is buoyantly supported to its uppermost position and, depending upon the orientation of the sensor, places the magnet adjacent the bottom of the reed switch or floats the magnet above the reed switch. When the liquid level drops below the predetermined value, the float is lowered and causes the opposite effect on the reed switch due to the repositioning of the magnet with respect thereto. The associated circuit monitors the opening and closing of the reed switch to control the illumination of a warning lamp if the level has dropped below the predetermined level. The primary disadvantage of employing the reed switch sensor of the '124 patent is due to the requirement that the entire sensor be mounted in a particular orientation, with respect to the vertical, in order that the float containing the magnet is properly aligned for responding to buoyant forces or the lack thereof.